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ing and radiating powers; the variations in the surface of the land, as forests, sandy deserts, verdant plains, rocks, &c., mountain-chains covered with masses of snow, which diminish the temperature; the reverberation of the sun's rays in the. valleys, which increases it; and the interchange of currents, both of air and water, which mitigate the rigour of climates; the warm currents from the equator softening the severity of the polar frosts, and the cold currents from the poles tempering the intense heat of the equatorial regions. To these may be added cultivation, though its influence extends over but a small portion of the globe, only a fourth part of the land being inhabited.
Temperature does not vary so much with latitude as with the height above the level of the sea; and the decrease is more rapid in the higher strata of the atmosphere than in the lower, because they are farther removed from the radiation of the earth, and being highly rarefied, the heat is diffused through a larger space. A portion of air at the surface of the earth, whose temperature is 70° of Fahrenheit, if carried to the height of two miles and a half, will expand so much that its temperature will be reduced 50°; and in the ethereal regions the temperature is 90° below the point of congelation.
The height at which snow lies perpetually de
short days in winter a frosty and clear night everywhere on a level
of the same
equally, and rad
surface rallel to
zones of e
to each pole
the altitudes of the su the internal heat of t
number of region from has probably state for thousands of heat, however, in the gular in all latitudes, where the isothermal through places of are parallel to turbance are very
greatest influence, acco we are indebted for th known on the subject continents, the distribut the surface of the globe,
IN OF THE
the poles, and is higher
w but it varies from Show rarely falls when the sphere dry. Extensive
their evaporation, and contrary, dry and warm es of the Andes, plains e leagues raise the temree or four degrees above same altitude on the rapid
sequently the line of
cording as one or other Aspect has also a great of perpetual snow is much outhern than on the northa mountains; but on the mean height between snow lies perpetually he level of the sea; he ground continuear the north pole.
vever, the cold is Sandwich land, s of latitude, e-sea-beach; the 53rd
tual snow descends even to the level of the
This preponderance of coud in the southmisphere cannot be altogether attributed to nter being longer than ours by so smail a ty as 7 days, even allowing to that its due ee; but it is probably owing to the open and the south pole, which permits the icedescend to a lower latitude by ten degrees y do in the northern hemisphere, on acthe numerous obstructions opposed to the islands and continents about the 2. Icebergs seldom float farther to the 1 the Azores; whereas those that come outh pole descend as far as the Cape of and occasion a continual absorption elting.
nce of mountain-chains does not d upon the line of perpetual conattract and condense the vapours air, and send them down in torrents adiate heat into the atmosphere at 1, and increase the temperature of › reflection of the sun's rays, and y afford against prevailing winds. ry, one of the most general and cold arising from the vicinity of ezing currents of wind which peaks along the rapid decli
creases from the equator to the poles, and is higher in summer than in winter; but it varies from many circumstances. Snow rarely falls when the cold is intense and the atmosphere dry. Extensive forests produce moisture by their evaporation, and high table-lands, on the contrary, dry and warm the air. In the Cordilleras of the Andes, plains of only twenty-five square leagues raise the temperature as much as three or four degrees above what is found at the same altitude on the rapid declivity of a mountain, consequently the line of perpetual snow varies according as one or other of these causes prevails. Aspect has also a great influence; the line of perpetual snow is much more elevated on the southern than on the northern side of the Himalaya mountains; but on the whole it appears that the mean height between the tropics at which the snow lies perpetually is about 15207 feet above the level of the sea; whereas snow does not cover the ground continually at the level of the sea till near the north pole. In the southern hemisphere, however, the cold is greater than in the northern. In Sandwich land, between the 54th and 58th degrees of latitude, perpetual snow and ice extend to the sea-beach; and in the island of St. George's, in the 53rd degree of south latitude, which corresponds with the latitude of the central counties of England,
perpetual snow descends even to the level of the ocean. This preponderance of cold in the southern hemisphere cannot be altogether attributed to the winter being longer than ours by so small a quantity as 7 days, even allowing to that its due influence; but it is probably owing to the open sea round the south pole, which permits the icebergs to descend to a lower latitude by ten degrees than they do in the northern hemisphere, on account of the numerous obstructions opposed to them by the islands and continents about the north pole. Icebergs seldom float farther to the south than the Azores; whereas those that come from the south pole descend as far as the Cape of Good Hope, and occasion a continual absorption of heat in melting.
The influence of mountain-chains does not wholly depend upon the line of perpetual congelation; they attract and condense the vapours floating in the air, and send them down in torrents of rain; they radiate heat into the atmosphere at a lower elevation, and increase the temperature of the valleys by the reflection of the sun's rays, and by the shelter they afford against prevailing winds. But, on the contrary, one of the most general and powerful causes of cold arising from the vicinity of mountains is the freezing currents of wind which rush from their lofty peaks along the rapid decli